Insulating fabrics are known which comprise an impermeable, thermally efficient sheet material such as neoprene rubber. Such fabrics are, however, not wearable next to the user's skin for extended periods of time, mainly due to the interference they can cause to the natural biological functions of the skin, in particular perfusion of the skin with oxygen and removal of natural excretions such as water vapour, salt, urea and carbon dioxide.
Previous efforts to improve the wearability of impermeable materials have included perforation of the material and lamination with a more skin-compatible material such as woven cotton.
British Patent No. 1267712, for example, describes. (FIG. 4) a breathable fabric in which a perforated elastomeric sheet is bonded between stretch-fabric sheets. The diameter of the perforations reduces slightly towards the outside of the finished garment to facilitate manufacture.
Such fabrics are reasonably wearable given normal external conditions and the biological functions of the wearer. However, if for example the wearer sweats or warms up during exercise or under stress, or the external temperature or humidity rises or falls, or the fabric becomes soaked with water, or in other abnormal situations, the breathable efficiency of the fabric declines rapidly, which can make the garment extremely uncomfortable or even dangerous to wear. Such poor adaptability has limited the use of breathable elastomeric fabrics, for example for insulating and/or protective garments, for medical or veterinary garments and/or dressings (where the patient's skin may be injured or prolonged close contact with the skin may be required), or for exercise and sports garments where rapid changes of perspiration and other skin functions take place. The present invention aims to provide a breathable fabric which goes at least some way towards overcoming the above disadvantages.
According to the present invention, there is provided a fabric comprising a sheet formed of a substantially impermeable material having perforations provided therethrough, each perforation having at least one relatively wide region and at least one relatively narrow region along its length to define an internal chamber open to a first ("inner") side of the sheet and sufficiently closed to the other ("outer") side of the sheet to permit air passing from the first to the other side of the sheet to accumulate in the chamber under increased pressure prior to passing out to the other side of the sheet.
The expressions "relatively wide" and "relatively narrow" mean that the respective regions are wide and narrow relative to each other. The expression "fabric" includes a fabric portion, and the expression "sheet" includes a sheet portion.
The substantially impermeable sheet may be a unitary sheet or a laminate, and is preferably elastomeric (e.g. formed from a rubber such as neoprene rubber). In the case of a laminate, different materials may if desired be used for different lamina so as to provide overall a sheet having the desired properties.
Closure of the chamber to the outer side of the sheet by a relatively narrow region of the perforation, in the resting condition of the sheet, may be complete or partial, and the materials and/or chamber configuration are suitably chosen so that on stretching and/or bending of the sheet or one or more particular lamina thereof the relatively narrow region opens wider than its resting condition to allow exchange of air between the two sides of the sheet. Stretching/bending so as to cause the relatively narrow region of the perforation to open typically results from the desired build-up of pressure in the chamber and/or by movement of the fabric in use.
The arrangement may also suitably be capable of creating a pumping effect in the chamber(s) by the periodic stretching and/or bending of the fabric in use, to assist the exchange of air between the inner and the outer sides of the sheet.
In general, it is preferred that even at its widest stretch the relatively narrow region of the perforation is no more than about 65% of the width of the relatively wide region, and less (most preferably substantially less) in the resting condition of the sheet, e.g. less than about 50%, more preferably less than about 35%, for example less than about 15%, of the width of the relatively wide region in the resting condition of the sheet. Where the sheet is a laminate, different lamina may optionally be of different flexibility, and suitably the lamina including the relatively narrow region of the perforation may be of greater flexibility than the lamina including the relatively wide region, for example through being thinner and/or of a material of greater elasticity.
The sheet may also include perforations of different configuration to those which form a novel feature of the invention, e.g. conventional straight-sided fully open perforations, or tapered perforations such as described in the prior art mentioned above. The sheet may also include unperforated regions.
The fabric may additionally have one or more permeable layers, e.g. of woven material, suitably bonded to the perforated sheet. The fabric is suitably capable of being rolled up for storage or transport.
According to a further feature of the present invention, therefore, there is provided a wearable article, such as a garment or dressing, formed from the novel fabric as defined above, the first side of the sheet formed of the substantially impermeable material suitably being directed to the inner side of the article as worn and the other side of the sheet suitably being directed to the outer side of the article as worn.
Each novel perforation as defined above defines at least one chamber within the sheet at the relatively wide region(s) of the perforation, the chamber(s) communicating to both sides of the sheet. One chamber is typically formed by a depression in that side of the sheet which is closer to the body of the user (the "inner" side), to partially enclose a volume of air directly above the user's skin.
The walls of each relatively wide and/or relatively narrow region of the perforations may suitably be parallel over at least a part of the length of the respective region. The perforations are preferably unbranched.
The perforations and associated chambers are suitably of sufficient size and spacing apart to permit the natural biological functions of the user's skin to continue substantially unhindered over a desired period of time, while permitting a controlled (but not excessive) retention of the user's body heat.
The components of the fabric should be non-toxic, non-irritant and comfortable to wear (in the sense of lightweight, flexible and soft to the touch), as well as being resistant to attack and degradation from all natural by-products of the user's body (e.g. sweat, blood, tissue fluid, urine, pus, and gases such as carbon dioxide).
It is found that fabrics of the present invention retain to a substantial extent the advantageous thermal properties of the impermeable sheet material while permitting to a surprising degree the natural biological functions of the user's skin to continue substantially unhindered.
Without wishing to be bound by theory, it is believed that the fabric of the invention permits the natural excretions to diffuse away from the user's skin and atmospheric oxygen to access the user's skin in an unexpectedly efficient way, because the relatively warm and moist air just above the user's skin collects in the chamber(s). This accumulation enables the air in the chamber(s) to attain a higher humidity, temperature and pressure than would be the case in a more open perforation. When, therefore, the relatively narrow region of the perforation opens due to the factors described above, the expulsion and replenishment of fresh air to the chamber takes place with enhanced speed and efficiency.
In more detail, it is believed that the perforations according to the present invention may mimic to some extent the properties of pores (which in biological systems cause an active or driven diffusion of molecules through a barrier with greater efficiency, for a given open area, than larger holes). However, the diffusion properties of the fabric according to the present invention are surprisingly enhanced. Accordingly, the relatively narrow region of the perforation should ideally not be so open that the perforation begins to function more as a hole than as a pore. This, in general, the diffusion rate should be dependent on the perforation diameter (as for biological pores) and not area (as for holes), and/or edge effects such as so-called "diffusion shells" should (as for pores) play a significant part in creating a relatively sharp concentration gradient immediately outside the perforation.
Fabrics in which the perforated elastomeric sheet is a laminate, the lamina including the relatively narrow region of the perforation being of greater flexibility than the lamina including the relatively wide region, and the relatively narrow region of the perforation lying closed or substantially closed in the resting condition, are particularly preferred. Such fabrics provide substantial protection to the wearer from cold or other external hazards (e.g. water, chemicals, bacteria, air etc), while permitting an enhanced air-exchange efficiency as soon as high levels of wearer activity arise, which cause the perforations to open due to flexing of the fabric and/or the higher temperatures and pressures within the chambers of the fabric. In such fabrics, the perforations may conveniently be interspersed with smaller numbers of other types of perforation according to the present invention and/or other (e.g. conventional) perforations. In one particular form, the perforations according to the present invention open when the vapour pressure of moisture in the chamber(s) reaches saturated vapour pressure.
The fabric of the present invention is preferably arranged to regulate the wearer's skin temperature to normal body temperature (37.degree. C.). This requires that the fabric functions as an insulator below body temperature and as a cooling medium above body temperature. The capacity of the perforations to open above a threshold chamber pressure and/or temperature can be exploited to permit increased evaporative loss due to sweating from the skin surface as the body temperature exceeds 37.degree. C., resulting in a skin temperature reduction through loss of latent heat of vaporisation from the skin. Thus, the fabric acts to cool the skin surface and maintain normal body temperature.
As the body temperature drops to 37.degree. C. the production of sweat ceases and hence the cooling effect diminishes. This is seen as a continuous process resulting in the maintenance of homoiothermic biological conditions under varying enviromental temperatures. The effect is more marked when the concentration of perforations is high enough to create conditions of significantly increased evaporation from the skin surface. If the fabric is to be used solely to elevate body temperature (e.g. for the initial treatment of victims of hypothermia), the concentration of perforations should be low enough to satisfy the requirement of sufficient removal of water vapour, but for many other uses the opposing warming (below body temperature) and cooling (above body temperature) effects should be maintained in proper balance depending on the desired end use.
By selecting particular elastomeric materials, particular lamina thicknesses, particular sizes of relatively wide regions of perforations, particular sizes of relatively wide regions of perforations, different concentrations of perforations over the fabric area and/or different arrangements of perforation types over the area of the fabric, the fabric's properties can be adjusted to suit the intended use. Moreover, by careful selection of materials and configuration, the fabric can be made to respond in its "breathability" to variations in external conditions and/or in the user's biological functions, so that to some extent such fabrics can self-regulate their "breathability" and hence automatically control the environment next to the wearer's skin within a pre-set temperature range.